The present invention pertains to a refrigeration system to provide the cooling requirements of an ethylene plant. More particularly, the invention is directed to the use of a binary refrigerant comprising a mixture of methane and ethylene for cooling in an ethylene plant.
Ethylene plants require refrigeration to separate out desired products from the cracking heater effluent. Typically, a C.sub.3 refrigerant, usually propylene, and a C.sub.2 refrigerant, typically ethylene, are used. Often, particularly in systems using low pressure demethanizers where lower temperatures are required, a separate methane refrigeration system is also employed. Thus three separate refrigeration systems are required, cascading from lowest temperature to highest. Three compressor and driver systems complete with suction drums, separate exchangers, piping, etc. are required. Also, a methane refrigeration cycle often requires reciprocating compressors which can partially offset any capital cost savings resulting from the use of low pressure demethanizers.
Mixed refrigerant systems have been well known in the industry for many decades. In these systems, multiple components are utilized in a single refrigeration system to provide refrigeration at a wider range of temperatures, enabling one mixed refrigeration system to replace multiple pure component cascade refrigeration systems. These mixed refrigeration systems have found widespread use in base load liquid natural gas plants. Articles have also been written on the application of mixed refrigeration systems to ethylene plant design but they are complicated in operation due to the multiplicity of components in the refrigerant. Also they are less efficient in the propylene refrigeration compressor cycle temperature range at -40.degree. C. or warmer.